Evolution of the Ca2+ current during dialysis of isolated bovine chromaffin cells: effect of internal calcium

Protein phosphorylation maintains the normal function of cloned human Cav2.3 channels

Ca_v2.3 Ca²⁺ channels are subject to cytosolic regulation, which has been difficult to characterize in native cells. Neumaier et al. demonstrate the role of phosphorylation in the function of these channels and suggest a close relationship between voltage dependence and the phosphorylation state.

R-type currents mediated by native and recombinant Ca_v2.3 voltage-gated Ca²⁺ channels (VGCCs) exhibit facilitation (run-up) and subsequent decline (run-down) in whole-cell patch-clamp recordings. A better understanding of the two processes could provide insight into constitutive modulation of the channels in intact cells, but low expression levels and the need for pharmacological isolation have prevented investigations in native systems. Here, to circumvent these limitations, we use conventional and perforated-patch-clamp recordings in a recombinant expression system, which allows us to study the effects of cell dialysis in a reproducible manner. We show that the decline of currents carried by human Ca_v2.3+β₃ channel subunits during run-down is related to adenosine triphosphate (ATP) depletion, which reduces the number of functional channels and leads to a progressive shift of voltage-dependent gating to more negative potentials. Both effects can be counteracted by hydrolysable ATP, whose protective action is almost completely prevented by inhibition of serine/threonine but not tyrosine or lipid kinases. Protein kinase inhibition also mimics the effects of run-down in intact cells, reduces the peak current density, and hyperpolarizes the voltage dependence of gating. Together, our findings indicate that ATP promotes phosphorylation of either the channel or an associated protein, whereas dephosphorylation during cell dialysis results in run-down. These data also distinguish the effects of ATP on Ca_v2.3 channels from those on other VGCCs because neither direct nucleotide binding nor PIP₂ synthesis is required for protection from run-down. We conclude that protein phosphorylation is required for Ca_v2.3 channel function and could directly influence the normal features of current carried by these channels. Curiously, some of our findings also point to a role for leupeptin-sensitive proteases in run-up and possibly ATP protection from run-down. As such, the present study provides a reliable baseline for further studies on Ca_v2.3 channel regulation by protein kinases, phosphatases, and possibly proteases.

Naloxone inhibits nicotine-induced receptor current and catecholamine secretion in bovine chromaffin cells

Nicotine-induced catecholamine (CA) secretion and inward ionic currents were inhibited by the opioid antagonist naloxone in cultured bovine chromaffin cells. Naloxone inhibited nicotine-induced CA secretion, as detected by an on-line real-time electrochemical technique, in a dose-dependent manner (IC(50)=29 microM). In voltage-clamped chromaffin cells, nicotine (10 microM) evoked an average peak inward current of -146 pA that was inhibited by low concentrations of naloxone (42% at 0.1 microM). The antagonist also inhibited total charge influx associated with nicotinic receptor activation (53% at 0.1 microM). This provides strong evidence that naloxone modulation of nicotine-induced CA secretion does not involve opioid receptors but results from the direct interaction with the nicotinic receptor itself, which might also be the case for other related opioid compounds.

Growth hormone-releasing hormone decreases voltage-gated potassium currents in GH4C1 cells

The electrophysiological properties of anterior pituitary somatotropes integrally involve the function of voltage-gated K+ currents. In this study, we have used GH4C1 cell lines to investigate the effect of human GHRH on voltage-gated K+ currents. Because of a clear 'rundown' of the K+ current with classic whole cell recording (WCR) without ATP in pipette solution, nystatin-perforated WCR was the major recording configuration used. Using a low Ca2+ (0.5 mM) bath solution containing Co2+ (1 mM) and TTX (1 microM), GH4 cells predominantly exhibited an outward delayed rectifier K+ current (IK). Local application of growth hormone releasing hormone (GHRH) (100 nM) reversibly reduced the amplitude of the K+ currents (to 83% of control). There was no effect of GHRH on the activation curve of the K+ current and no difference observed using 2.5 mM Ca2+ or low Ca2+ (0.5 mM Ca2++1 mM Co2+) bath solutions. Under the condition of low Ca2+ bath solution, the application of apamin (1 microM) or charybdotoxin (1 microM), two specific blockers of the Ca2+-activated K+ current, did not alter the K+ current or the response to GHRH. This reduction in the K+ current by GHRH was also observed with classic WCR with a pipette solution containing ATP (2 mM). The GHRH-induced reduction in the K+ current was completely abolished by the presence of GDP-beta-s (500 microM) in the pipette solution or by addition of PKC inhibitors, calphostin C (100 nM) and chelerythrine (1 microM), in bath solution. Inhibitor for cAMP-PKA system (Rp-cAMP and H89) did not affect the K+ current response to GHRH. These results suggest that GHRH reduces the voltage-gated K+ current in GH4C1 cells, a response that is mediated by G-proteins and PKC system but not by cAMP-PKA system. The reduction in the K+ current may partially contribute to the GHRH-stimulated growth hormone (GH) secretion.

ATP and G proteins affect the runup of the Ca2+ current in bovine chromaffin cells

The Ca2+ current recorded by the whole-cell technique in chromaffin cells shows, before the often described rundown, a transient facilitation or runup. Initial current amplitude was 570 +/- 165 pA and then it increased by 49 +/- 23% (n = 19, SD) over 2 +/- 1 min in the absence of adenosine 5'-triphosphate (ATP). In the presence of ATP, this process occurred with the same magnitude but it was slowed in a dose-dependent manner, lasting 17 +/- 2 min with 2 mM ATP (n = 8). Since adenosine 5'-diphosphate (ADP) does not reproduce this ATP effect, a complex series of phosphorylations is likely to intervene and we show that, at least, a cAMP-dependent i.e., cyclic adenosine monophosphate) phosphorylation occurs. Pertussis toxin (PTX) pretreatment yielded an already maximal Ca2+ current (around 1000 pA) at the time of the patch rupture, which only slightly increased thereafter (10%, n = 11). Also, guanosine 5'-diphosphate (GDP) and guanosine 5'-O-(2-thiodiphosphate) (GDP[ beta s]), induced a fast runup, which was absent in the presence of GTP. Furthermore, we show that facilitation does not occur in the presence of dihydrophyridine (DHP) antagonists. Globally, our data suggest that an ATP-dependent phosphorylation stabilizes the inhibitory control exerted by a PTX-sensitive G protein and, as a result, slows down the facilitation of L-type Ca2+ channels. The recruitment of L-type channels can also be facilitated by the application of a DHP agonist or a depolarizing prepulse protocol.l We show that these processes are only effective over a period which parallels the runup and are not additive to it.(ABSTRACT TRUNCATED AT 250 WORDS)